Passive Inter-Modulation (PIM) interference has been a problem in cellular networks since their inception. A PIM interference problem occurs when energy from a transmission in one frequency band leaks into another frequency band, resulting in interference. It typically occurs where a high-power transmitter at the cellular site that is transmitting signals over the air to mobile devices (i.e., in the downlink direction) on one frequency band is located physically very close to a sensitive receiver at the cellular site that is receiving signals transmitted over the air by mobile devices (i.e., in the uplink direction). In an ideal world, the uplink and downlink transmission would not affect one another because they use different frequency bands. However, in the real world, a small leakage of energy from the downlink transmission into the uplink transmission can occur that completely drowns out any legitimate communication to the base station from a user's mobile device.
There are many causes of PIM interference, but the causes are generally electrically non-linear elements in the radio frequency (RF) pathway that occur unintentionally. Examples of causes include, for example, corroded connectors, poor quality cables, joints between two dissimilar metals, moisture, etc., that act (unintentionally) as electronically non-linear elements in the RF chain. In the multi-carrier systems and networks that underpin both four generation (4G) and many emerging fifth generation (5G) wireless standards, such electrically non-linear elements cause different frequency components that are present in the downlink and uplink signals to mix and multiply with one another, producing new frequency components. These new frequency components result in PIM interference, both inside and outside of the allocated frequency bands.
FIG. 1 illustrates a block diagram of a traditional cellular site 2 for which existing PIM test solutions are used to perform PIM testing. The existing PIM test solutions typically involve a technician visiting the cellular site 2 to perform PIM testing, taking a sector or an entire base station 3 offline, connecting a test instrument 4 to connection points of a transmitter 6 and a receiver 7 of the base station 3, using the test instrument 4 to inject a high-power signal into the RF path of the transmitter 6, and taking measurements with the test instrument 4 to determine whether any energy from the injected high-power signal has leaked into the RF path of the receiver 7. A coaxial cable 8 connects the base station 3 to one or more antennas 9 disposed on the top of a tower 11. The RF path or chain of the cellular site 2 includes the transmitter 6, the receiver 7, the coaxial cable 8, the antenna 9, and any RF connectors or other components disposed in the electrical pathway between the transmitter 6 and the antenna 9 or between the antenna 9 and the receiver 7. Any electrical nonlinearities along the RF path can result in PIM interference problems.
The disadvantages of the typical PIM test solution are that it requires taking sectors or the entire base station 3 offline and thereby disrupting service and that the costs associated with having a technician visit the cellular site 2 is very expensive. Other disadvantages include limitations on the number of uplink connections that can be analyzed, the inability to detect gradually deteriorating performance without having to make multiple visits to the celluar site, and the possibility of not detecting PIM interference that is intermittent and not present at the time of the visit. For example, moisture may enter on RF connector when it rains, leading to a PIM interference problem, but the PIM interference problem may dissappear when the moisture evaporates. Consequently, the PIM interference may only be detactable by a technician immediately after a rain event before the moisture has evaporated.
Yet another disadvantage is that all-digital cellular sites that do not have an accessible connection point for the test instrument cannot be tested using the typical PIM test solution. FIG. 2 illustrates a block diagram of an all-digital cellular site 12, which may be, for example, a Common Public Radio Interface (CPRI) cellular site. With an all-digital cellular site of this type, a data center 13, sometimes referred to as a baseband pool or a baseband hotel, communicates via an optical fiber link 14 with the tower 15 of the site. The data center 13 may serve multiple all-digital cellular sites (not shown) and is typically remotely located relative to the locations of the cellular sites, e.g., ten to twenty kilometers away from the cellular sites. The optical fiber link 14 often runs to the top of the tower 15 or part way up the tower 15 before interfacing with optical-to-electrical conversion equipment and the RF chain (i.e., transmitter, receiver, power amplifiers, coaxial cables, etc.). The box 16 represents the optical-to-electrical conversion equipment and the RF chain, which is electrically coupled to the antenna 17, typically with coaxial cable (not shown). In such networks, typical PIM testing solutions such as those described above are unsuitable for performing PIM testing because there is no accessible connection point for interfacing the test equipment with the RF chain.
A need exists for a system and method for detecting PIM interference in cellular networks that do not require the base station or any sectors to be taken offline to perform PIM testing, that eliminate the need for a technician to make a visit the cellular site, that can test multiple uplink connections simultaneously, that can be used to test all-digital cellular sites that do not have an accessible connection point to the RF chain, and that can detect gradually deteriorating performance due to PIM interference and intermittent PIM interference problems.